It is known that polyimides have excellent properties in terms of heat resistance, mechanical characteristics, electrical characteristics, weatherability and the like and are useful as materials for films and moldings. For example, a polyimide prepared from 4,4'-diaminodiphenyl ether and pyromellitic acid dianhydride can produce films and moldings having excellent heat stability, and these products are widely used in electrical insulation and the like.
In the field of heat-resistant synthetic paper, on the other hand, aramid synthetic paper is known and is widely used. However, with the improvement in advanced materials for use in spacecraft and aircraft, great interest has recently been directed toward the development of synthetic paper products having excellent heat resistance and strength which are superior to those of aramid paper.
Attempts have been made to obtain polyimide paper by subjecting a polyimide fibrid material to paper making. For example, JP-A-62-297330 discloses a process for the production of a polyimide fibrid material in which a poly(amic acid), which dissolved in an aprotic polar organic solvent, is injected into a coagulation bath containing acetic acid and/or acetic anhydride under high speed agitation. Further, JP-A-64-26718 discloses a process for the production of a polyimide fibrid material in which a poly(amic acid) solution is injected under high shear force into a coagulation bath containing a mixture comprising a chemical cyclization agent and a cyclization catalyst. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, polyimide paper made of the polyimide fibrid material obtained by such processes has a problem in terms of its practical use because of poor strength.
In addition, JP-B-60-1402 discloses a process for the production of pulpy particles in which a polymer solution prepared by solution polymerization in the presence of staple fibers is dispersed in a precipitant. (The term "JP-B" as used herein means an "examined Japanese patent publication".) According to this process, pulpy poly(amic acid) particles containing staple fibers are obtained. However, this process has a problem of causing a reduction in polymerization degree, because an aprotic polar solvent remains inside the pulpy particles, and the poly(amic acid) is apt to be hydrolyzed by the remaining aprotic polar solvent. In addition, the aprotic polar solvent that remains inside the pulpy particles in a large amount causes other problems in that a high purity polyimide paper cannot be obtained when the pulpy particles are subjected to paper making and imidization. In particular, the product becomes a film-like paper with no appearance of pulpy particles when pressurized after paper making because of the remaining aprotic polar solvent which oozes out due to the applied pressure and dissolves the pulpy particles and causes mutual adhesion of the particles.
With the recently increasing demand for materials having improved mechanical properties and heat resistance, complexing of polyimide resins and other materials has been proposed, and processes in which polyimide resin powder and staple fibers are mixed and molded in the same manner as the case of polyimide moldings have been examined. Polyimide moldings are produced by subjecting polyimide resin powder to press molding and heat treatment, as disclosed for example in JP-A-2-18420, JP-B-49-5737, JP-A-61-250030 and JP-A-1-292035. It is difficult, however, to obtain a composite material having high strength because of the difficulty in uniformly mixing and dispersing the resin powder and staple fibers.